Recent research has shown that usage of the radio spectrum is often fairly inefficient. One key factor in this is the current spectrum licensing system. That is, some part of the radio spectrum is licensed to a party, such as an operator of a radio communications system, who is given an exclusive right to use this part of the radio spectrum. For example, even though most of the spectrum relevant for mobile communication, is licensed, several measurements (see, e.g. T. Erpek, K. Steadman, D. Jones, “Spectrum Occupancy Measurements: Dublin, Ireland, Collected On Apr. 16-18, 2007”, Shared Spectrum Company Report, 2007) indicate that some parts of the spectrum are highly underutilized. Therefore, a more flexible use of the radio spectrum has become a research intensive subject within which the aim is at optimizing, i.e. maximizing, the usage of the available radio spectrum.
To address the above-indicated issues, the FCC (Federal Communications Commission) took, in 2005, an initiative to open up for so-called secondary use of the spectrum. That is, radio resources that, e.g. under a license, are owned by one party—the primary party—may also be used by others (secondary users) for purposes (secondary use) that do not fall within the operation (primary use) of the primary party. Consequently, a secondary user may use radio resources owned by the primary party without having a license and without having made an explicit agreement with the primary party. A requirement for accessing radio resources as a secondary user is that the primary party should not be exposed to harmful interference caused by the secondary use. In brief: a secondary user is allowed to transmit over radio resources owned by the primary party if it can be ensured that the primary party is, for all practical purposes, not affected.
Consequently, a primary use of radio resources is a use that takes precedence, normally because the use falls within the activities performed or expressly allowed by the primary party owning the radio resources. A primary user (e.g. the primary party) is a user that engages in a primary use of radio resources. A primary user equipment is a technical equipment capable of engaging in a primary use of radio resources. Correspondingly, a secondary use of radio resources, which have a primary use, is a use that can be allowed provided the use does not interfere with any primary use. A secondary user is a user that engages in a secondary use of radio resources. A secondary user equipment is a technical equipment capable of engaging in a secondary use of radio resources. Notice that a particular piece of user equipment may be a primary user equipment as well as a secondary user equipment (multiple capabilities).
As is well understood by the person skilled in the art, the particular manner in which the radio resources involved here are defined depends of course on the situation under consideration, and in particular on how radio resources are defined for the primary use. In general, however, the radio resources involved are normally defined in terms of at least spectrum and time utilization. Specification of communication codes, e.g. channelization codes, scrambling codes etc. may also be part of defining the radio resources that can be employed for secondary use. Furthermore, geographical limitations may provide further means of definition. As is well understood by a person skilled in the art, radio signals are subject to distance dependent attenuation. That is, the power density of a radio signal decreases rather rapidly with a distance to a source of the radio signal. Hence, sufficiently far from the signal source, the signal power can be neglected and the spectrum used to transmit the radio signal can be used simultaneously for transmitting other radio signals. For example, the same radio channel can be reused in several different locations, provided that the geographical distance between these locations is sufficiently large. Consequently, the same radio channel can be viewed as distinct radio resources in different geographical regions. This is the principle underlying frequency reuse in, for example, cellular communications systems.
As a consequence of the above-mentioned FCC initiative, several standardization activities in the area of dynamic spectrum sharing have been initialized (IEEE 802.22, IEEE SCC41/P1900), and some are about to be started (ETSI activities). Regulators are currently investigating the possibilities of allowing secondary use of the spectrum.
According to one suggested scenario, access of a secondary user is performed based on so-called sensing of radio resources owned by the primary party. Here, sensing is the act of detecting whether the radio resources intended to be utilized in a secondary use are already utilized in a primary use or a secondary use by another secondary user. Only if the radio resources are not utilized in the primary use, is it allowed to utilize the radio resources for the intended secondary use. One way to accomplish this is described in Lifeng Lai, Hesham El Gamal, Hai Jiang and H. Vincent Poor, “Cognitive Medium Access: Exploration, Exploitation and Competition”, submitted to IEEE/ACM Trans. on Networking, October, 2007. Here, transmissions relating to both primary and secondary use are performed in a time slotted manner based on a common synchronisation scheme. A first part of each time slot is dedicated for primary use. Secondary users may sense this part, and if a transmission is detected in this part, it can be safely concluded that the transmission relates to a primary use, in which case secondary use is not possible. A last part of the time slot is only accessed for secondary use if no primary use is detected during the first part of the time slot. The secondary users may follow a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) scheme to share the resource fairly.
Above, the secondary users need to synchronize their timeslots in order to be able to distinguish between primary and secondary uses. This is a drawback since extra signalling among the secondary systems will be required. Furthermore, one may easily envision situations where a secondary user equipment that is out of synchronization with other user equipments (primary and/or secondary) will be detected as a primary user equipment (e.g. during the first “sensing”-part of the timeslot) and will hence only need to share the radio resources with the primary users. This would result in an unfair sharing of the opportunities for secondary access among the secondary users.
A problem to be solved by the present invention is thus to overcome or at least mitigate at least one of the above-indicated difficulties relating secondary use of radio resources.